Depth capture is a phenomenon where a stereoscopically viewed surface captures a part of background texture to the same depth plane as the surface itself (Ramachandran & Cavanagh, 1985). Although this phenomenon has repeatedly been demonstrated in laboratory settings, it has not been observed in natural settings. We describe a very salient depth capture in a waterfall illusion located in Helsinki, Finland. The illusion consists of a waterfall and a bridge 40 meters in front of it. When viewed from a distance of 300 meters, the bridge captures the flowing water to the same depth level with it.

We recorded the waterfall scene to a video tape and showed it to experimental subjects in a laboratory setting. The subjects viewed the video and used a map to estimate the depth positions of the waterfall and the bridge. The results consistently showed that depth capture was detected.

The waterfall capture includes several interesting features. Firstly, it could be assumed that a static bridge and a rapidly flowing water can very easily be segmented as different surfaces. However, for some reason this did not happen in this setting. Our hypothesis is that the brightness of the bridge and the flowing water were so similar that the differences in texture type and movement did not provide enough depth segmentation cues. Secondly, the depth capture in this context was completely different from stereoscopic capture which is based on rematching in a periodic texture (Häkkinen, Liinasuo, Kojo & Nyman, 1998). The rematching did not cause the waterfall capture because the observation distance was not suitable for stereopsis and the surface texture of the water was not periodic. Thus, the waterfall capture represents a natural occurrence of depth capture without stereopsis.